1. Field of the Invention
The present invention relates to an image forming apparatus of transfer type such as a copying machine, a laser beam printer (LBP) and the like, having a transfer means for transferring a toner image onto a transfer material, which toner image was formed and born on an image bearing member such as an electrophotographic photosensitive body, an electrostatic recording dielectric body, a magneto-recording magnetic body and the like by means of an appropriate image forming process means such as an electrophotographic process, an electrostatic recording process, a magnetic recording process and the like, and a fixing means for fixing the toner image onto the transfer material.
2. Related Background Art
FIG. 4 schematically shows an example of an image forming apparatus of the transfer type.
In this example, an image bearing 101 is constituted by a rotary drum-shaped electrophotographic photosensitive body. The photosensitive body 101 is rotated at a predetermined peripheral speed (process speed) in a clockwise direction shown by the arrow and is uniformly charged with predetermined polarity and potential by means of a primary charger 102.
Then, the charged surface is illuminated by image exposure light 103 from an image exposure means (for example, a projection exposure apparatus for an original image or a scanning exposure apparatus using image-modulated laser beam), with the result that potential of an exposed bright portion is reduced to form an electrostatic latent image corresponding to the exposure image information on the surface of the photosensitive body.
Then, the electrostatic latent image is subjected to normal development or reversal development by means of a developing device 104, thereby forming a toner image.
The toner image is transferred onto a transfer material S by a transfer means 105 at a transfer position (transfer portion) T. In this example, the transfer means 105 comprises a transfer charger having a discharge opening portion opposed to the photosensitive body 101 with a predetermined gap therebetween (non-contact type). The transfer position is defined between the photosensitive body 101 and the transfer charger 105.
The transfer materials S are supplied and separated one by one from a sheet feeding cassette (not shown), and the separated transfer material is sent to the transfer position at a predetermined control timing through a pair of registration rollers 106 and a transfer guide 107. The pair of registration rollers 106 serve to feed the transfer material S to the transfer position T at a timing such that a leading edge of the transfer material S just reaches the transfer position T when a leading edge of the toner image formed on the surface of the rotating photosensitive body 101 reaches the transfer position T.
After the leading edge of the transfer material S reaches the transfer position T and before a trailing edge of the transfer material S is passed by the transfer position T, a predetermined voltage is applied to the transfer charger 105 so that a back surface of the transfer material S being passed through the transfer position T is charged with polarity opposite to charging polarity of the toner. As a result, the transfer material S is electrostatically adhered (electrostatic adsorption) to the surface of the rotating photosensitive body 101 to be conveyed through the transfer position T; meanwhile, the toner image formed on the surface of the photosensitive body 101 is electrostatically transferred onto the surface of the transfer material S.
The transfer material S electrostatically adhered to the surface of the rotating photosensitive body 101 and left from the transfer position T is subjected to electricity removal by means of a separation charger 108, with the result that the transfer material is separated from the surface of the rotating photosensitive body 101. The back surface (non-imaged surface) of the transfer material S is separated from the surface of the rotating photosensitive body 101 and is contacted with and supported by a transfer support portion 110 disposed at a transfer material outlet of the transfer position T and is then sent to a convey belt device 111, where the transfer material is rested on a convey belt 112 with the toner-imaged surface facing upwardly.
After the transfer material is separated, transfer residual toner and foreign matters remaining on the surface of the rotating photosensitive body 101 are removed by a cleaning device 109 to clean the photosensitive body for preparation for next image formation.
The convey belt 112 of the convey belt device 111 is an endless belt mounted around and between a drive roller 113, a driven roller 114 and a tension roller 115 with predetermined tension, and an upper belt portion (upper run) of the convey belt on which the transfer material S rests is rotated at a predetermined peripheral speed from the transfer position T toward a fixing device 118. A suction fan 116 is disposed inside of the endless convey belt 112 to generate an absorbing force directing from the upper belt portion toward the interior of the convey belt. The transfer material S resting on the upper belt portion of the convey belt 112 is adsorbed and held by the upper belt portion under the action of the suction fan 116, so that the transfer material is conveyed by the shifting movement of the belt through a convey support portion 117. Then, the transfer material is shifted from the convey belt device 111 toward a fixing inlet guide 119 of the fixing device 118 to enter into a fixing nip portion (fixing position) N between a fixing roller 120 and a pressure roller 121, where the toner image is fixed to the transfer material. Thereafter, the transfer material is discharged onto a sheet discharge tray 124 by pairs of sheet discharge rollers 122, 123.
The convey support portion 117 defines a point where the transfer material S is separated from the convey belt 112, which point is substantially the same as a point where the convey belt 112 contacts with the drive roller 113.
The fixing inlet guide 119 supports the back surface of the transfer material S and serves to direct the leading edge of the transfer material into a transfer material inlet of the fixing nip N, and a downstream (in a transfer material conveying direction) upper end of the fixing inlet guide constitutes a fixing inlet support portion for defining a transfer material inlet height of the fixing device 118.
In this system, in order not to transmit shock (fixing entering shock) generated when the leading edge of the transfer material enters into the fixing nip N to the transfer position T, the convey support portion 117 is located below a line connecting between the upper end (fixing inlet support portion) of the fixing inlet guide 119 and an upper end of the transfer support portion 110, so that the transfer material S is curved downwardly and is strongly adsorbed by the convey belt 112 under the action of the suction fan 116, thereby preventing transfer deviation due to entering shock of the leading edge of the transfer material into the fixing nip N.
Further, a distance between the fixing nip N and the transfer position T is selected to be equal to or greater than A4R length (293 mm) so that possibility of shock is eliminated regarding transfer materials having half sizes (203 to 210 mm) such as A4 size and LTR size which have high frequency of usage.
Further, in order to prevent transfer deviation due to leaving shock (registration leaving shock) generated when the trailing end of the transfer material leaves the pair of registration rollers 106, the adsorbing force of the suction fan 116 is selected to be greater, and a relationship between speeds is selected as follows:
fixing speed.ltoreq.convey belt speed PA1 so that the transfer material is not pulled and the adsorbing force is increased, thereby preventing the leaving shock.
= registration roller speed
However, in the above-mentioned conventional example, since the distance between the transfer position T and the fixing nip N must be greater, a width of a main body of the image forming apparatus is increased, and, since the adsorbing force of the fan must be stronger, cost of the convey portion is increased.